It is often desirable to quantitatively measure the motion of a structure. This can be carried out using various instruments. A particularly useful family of said instruments uses optical techniques.
U.S. Pat. No. 4,619,529, issued Oct. 28, 1986, "Interferometric contact-free measuring method for sensing motional surface deformation of workpiece subjected to ultrasonic wave vibration", teaches a method for observing sound waves on a workpiece by interfering two beams that are reflected from different points on the workpiece. The pulsing relates to the pulsed laser beam that generates the sound waves on the workpiece. The interferometer beams are not pulsed.
U.S. Pat. No. 3,572,936, issued Mar. 30, 1971, "Stroboscopic Interferometric Holography", and U.S. Pat. No. 4,999,681, issued Mar. 12, 1991, "Real-time holographic interferometry with a pulsed laser and flicker-free viewing", teach two methods for producing a hologram of a vibrating object by using stroboscopic illumination. The motion is determined from the generation of a second reference hologram of the rotated object or the object at rest. The second hologram is produced as a second image of the first hologram to produce a interference that determines the motion.
A publication by O. Kwon et al, Opt Lett 12: (11) 855-857 November 1987, teaches a method of pulsed source interferometry. A conventional interferometer is equipped with a Q-switched Nd: YAG laser which is capable of generating high intensity pulses. This is necessary since the authors use only a single pulse to record the interferometric fringe pattern with a camera. The limitation to using a single pulse is overcome by the use of a grating to generate three interferograms which are acquired simultaneously by three cameras. In addition two gratings are used to generate the required phase shifts, restricting the source to be monochromatic due to the inherent chromatic dispersion of a grating.
A publication by S Nakadate et al.,Opt Acta 33: (10) 1295-1309 October 1986, teaches holographic interferometry. There are many other systems which do the same. In these methods, the contours of vibration amplitude are given as a fringe pattern.
One particularly useful device is based on optical interference using, e.g., a Michaelson Interferometer.
For example, an optical profiler is available from the company WYKO, under the name of WYKO RST Plus Optical Profiler. This is a scanning imaging white light interferometer. A block diagram of the device has the structure shown in FIG. 1. An incandescent light source 100 is focused through lens 102 to half mirror 104. The light is reflected down to a microscope 110. The light passes through microscope objective 112, to a beam splitter 114. The beam splitter 114 produces a reference beam 116 that is reflected to eventually recombine with the reflected object beam.
The object beam 118 passes to the object being imaged at 120, and is reflected. This beam then recombines with the reference beam 116, to produce an interference. A CCD video camera 125 images the operation. This system has the ability to detect minute features on the surface of the sample 99.